Heat the olive oil in a large Dutch oven; add the onion and fennel, sprinkled with a big pinch of salt and pepper to taste. Cook over medium heat, stirring often, until soft and golden but not brown, 20 minutes.,Add the tomatoes and their juice, broth, blood orange juice and a pinch of ground red pepper. Lower heat to a simmer; cook covered until the fennel is entirely soft and the tomatoes have released their juice and turned dark, about 30 minutes.,Allow the soup to cool slightly; puree until smooth in a blender in batches or with an immersion blender. Return the soup to the heat; heat to a simmer. Taste; adjust seasoning with additional salt and pepper if needed. Add a touch more broth if it seems too thick.,Serve in shallow bowls garnished with a pinch of grated zest, a spoonful of the yogurt and sprigs of fennel fronds.,______